The use of downhole motors in drilling operations has had limited success due to the short functional life span of drill bits. The life of a drill bit is related to the rotational speed of the drill bit and, heretofore, downhole motors arrangements have tended to rotatably drive the drill bit at relatively high rotational speeds. As a result, premature drill bit failure would result and, each time a drill bit would become worn, valuable drilling hours would be lost as the drill string would be removed from the borehole, the bit replaced and the drill string reinserted into the borehole. While the solution, reduction of the rotational speed of the drill bit, has been recognized for several years, downhole drill bit drive tools have nevertheless failed to meet expectations primarily because of drive shaft bearing failure and inadequate torque transmitting capability. Typically, a tubular bearing housing would be secured to the downhole end of the drill string to house a drill bit drive mandrel. Radial and thrust bearings would be interposed between the housing and mandrel to transmit radial and axial loads between the housing and mandrel during drilling operations and while raising and lowering the drill string from and into the borehole.
By way of background and as is well known in the this field, a drilling fluid, or "drilling mud" as it is known in the field, is pumped under pressure down the interior of the drill string to the drill bit and the fluid together with cuttings are returned to the surface along the exterior of the drill string. The fluid pressure at the bottom of the bore hole is considerable.
In one prior downhole drill bit drive arrangement, a controlled amount of drilling mud was allowed to pass through the bearing assemblies. However, the drilling mud was found to cause premature failure of the bearings and, accordingly, this approach was deemed an unsatisfactory solution to the problem. Various attempts at sealing the bearing and speed reduction mechanism have had little success. In addition to these difficulties, there still existed the problem of fitting a speed reduction mechanism within the constraints of a tubular housing which had substantially the same outside diameter of the drill string while being capable of meeting the high torque requirements.
Apart from the problems related to the presence of drilling mud in the bearings, the axial bearings of prior art devices have tended to fail prematurely because of inadequate load carrying capacity and/or unequal load distribution. Thus, one of the axial bearings would be subjected to a greater load than others of the bearings and fail first. Once this occurred the remaining bearings would fail soon thereafter.